We report an investigation of wetting of well-defined hydrophobic nanochannels prepared by silanization of mesoporous ordered silicas (SBA-15 type) with Rpore ~2-4 nm. Wetting in these systems can not be described by the classical models. Water occupies only fraction (30-60%) of the pore volume and the values of pressure for the pore filling are ~60-90% greater than predicted by the Laplace equation. The results are explained well using Derjaguin's model of wetting films and the concept of “disjoining pressure”. It is suggested that non-wetting fluid (water) is separated from the hydrophobic walls by thin film of the vapor phase (wetting fluid). The thickness of these films is estimated ~0.3-0.4 nm, or ~10-20% of the pore radius.
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